Ampeg SVT static/crackling on power up

[corliss1]

Hey hey! Schematic attached.

The amp works, sounds, and operates properly. It has a fresh recap and service, everything is cleaned, all that jazz. All voltages are correct. However, during the first 60-90 seconds, there is a lot of crackling noises, which would be more than enough to scare off a buyer testing it out in a store. After a few minutes, the amp settles down and operates fine.

I've isolated the issue to pin 6 of V2. Pulling v2 completely stops the noise, v3 has no effect on the crackle. Every tube in v2 causes the noise, so it's not a tube issue. Removing just pin 6 of v2 from the board also stops the noise. Running a clip lead from that pin back to the rest of the amp brings the noise back.

I have swapped the v2 socket.

Muting the AC of each pin on the socket with a cap to ground doesn't stop the issue, even on pin 6. The same goes with any of the power supply points, like R52 or D11 and whatnot.

Is there maybe a board issue around that socket that is causing arcing or warming up or something? I'm not aware of any more isolation I can do besides what I've noted.

Thanks for any ideas!

[g1]

Good to see you here RJ.
Assume you mean V2 in preamp not power amp? And how did you narrow it down to that half of V2?

[corliss1]

I'm so sorry I didn't clarify - v2 of the power amp page on the schematic.

[g1]

Ok. I'll still ask my other question, since AC grounding (via cap) did not help for any pin on that tube, how did you isolate the issue to pin 6 ?

[corliss1]

I replaced that tube socket since it was REAL loose anyway, and just lifted leg 6 so it wasn't making a connection. I also broke a trace on the board itself. That's the only pin that seems to exhibit the behavior, and the only thing that stops it is that.

[corliss1]

Actually, maybe I should phrase it this way - I suppose I technically *have not* isolated it to pin 6, but lifting that pin does resolve the issue, so it's at least that half of the triode.

I keep a couple 12AX7s (yeah, I know, an AU7 would be closer for testing...) with each triode's pins cut out for weird testing, which also confirms the issue to that half.

[g1]

pin 6 is one of the last things before the 'top half' power tube grids, so it could still be prior to that.
Signal routes through the cathode, but with the plate disconnected the cathode is pretty much dead. Also, the cathode is setting the bias, so with that tube not running, I would think it goes full cold bias and kills all sound from top half.
I guess with V2 and V3 in, but V1 removed, the noise is still there?

[corliss1]

Correct - V1 and V3 don't change anything at all. I was thinking if it was "before" that, muting pin 7 would have done something, but nope.

Any/all tube but v2 in and we're good.

[g1]

Did you have any luck with this one, or is it still in progress?

[corliss1]

Well, I called the owner, had him listen to it as it, and he didn't think it was a big deal at all. I ran it for *6* hours once, and shorter periods several other times just to be extra sure everything was stable, and no crackling came back, so he was happy with it how it was. But yeah, the Forum outage kinda hit right at the wrong time for this thread

[nevetslab]

On the most recent production amps from Ampeg, I too have been dealing with this issue. The first one I spent a good deal of time in the preamp, replacing the film coupling caps (C6 to Grid of V2 pin 3, C26 from cathode of V2 pin 3) around V2 in the Preamp stage, Resoldered the PCB header at J3 and receiving header on power amp PCB J11. None of those cured it. Initially, dead-patching Power Amp Input jack would mute the noise, then after also dead-patching Slave Out jack, powering down, waiting for caps to drain off, then powering back up, the static noise would stop. I ended up finding swapping V2 with V1 in the preamp stopped the noise. Days later, found the problem again. I had this issue with at least four recent production SVT-CL amps. Haven't had this problem on our older SVT-CL amps in our rental fleet (around 14 or so).

Another common issue after switching out of Standby, is DC charge remaining on C26 coupling cap from the cathode follower tube V2 in the preamp. I normally have the Master Volume turned CCW when switching out of S/B. Turning it up immediately after powering up initially, I'd get DC noise on the pot until that cap had fully discharged. Then no more noise on the Master.

I haven't found any issues on this loud static noise after coming out of S/B with either Driver Tube as mentioned above. They are isolated by V7 Power Amp's input tube. And haven't found any issues with V7 either, though early on in trouble-shooting the problem, I had swapped out V7 with new tube. Problem appears to remain coming from the preamp. So, the initial re-capping around V2 in the preamp wasn't fruitful.

[corliss1]

Interesting, nevetslab. I had a guy contact me about one of the newer ones as well. Needs all the tubes and *all* the pots are sheared off from it sliding around the back of a trailer. it may just not be worth it to repair just based on those items plus the labor cost alone, but we'll see!

[nevetslab]

Interesting, nevetslab. I had a guy contact me about one of the newer ones as well. Needs all the tubes and *all* the pots are sheared off from it sliding around the back of a trailer. it may just not be worth it to repair just based on those items plus the labor cost alone, but we'll see!

These SVT amps, whether it being an SVT-CL, SVT-AV, SVT-VR...ARE well worth restoring.

As I maintain a fleet of some 16 SVT-CL's, 11 SVT-VR's, and an SVT-AV, there are some simple modifications I've made to the amps to keep them road-worthy. On the Hold-down Clamps, I've added hi temp silicon rubber tubing slit to form a cushioned 'hat' on the cap of the hold-down clamp, and also added larger dia silicon rubber tubing to fit over the 'wings' that center the hat around the top of the tube, and a longer one that fits over the spring/tab joint so it no longer digs a hole thru the glass tube over time, killing the tube when it succeeds in penetrating the vacuum bottle. All this forms a suspension for each power tube, and vastly cut down on tube breakage on tours with the amps always inside a properly cushioned road case.

Also replacing cheap Chinese M6 or 1/4-20 cage nuts with proper US produced cage nuts and adding split-lock washers under the head of the chassis mounting screws, (between screw head and flat washer, which is all Ampeg has ever provided. Amps coming back off the road prior to my adding the split lock's, they would be lacking most of the chassis mtg screws, and all would be loose. Power and output transformer mounting hardware constantly loose, replacing with #10-32 with split lock and flat washers to lock them down. Xfmr core bolts usually loose as received new out of the box. The three M3 Standoffs (or earlier, #6-32) hex standoffs between chassis and front power amp PCB mounting hardware loose where the PHMS on top of chassis threaded into the standoffs are loose. The one on the side of the Power Supply caps IS THE GROUND REFERENCE PATH TO THE CHASSIS! Amp will hum if that's loose, or worse. HAS TO BE TIGHT!

I cut off the clear vinyl garden hose insulating harness tubing between the power amp chassis and preamp chassis, and replace it with Tech Flex, which allows easy positioning of both chassis to be reconnected and operated outside the chassis on the test bench. I also use the amp chassis (on SVT-CL's) as the power tube test fixture when matching tube for best groupings of the upper and lower tubes. For the SVT-VR, I had produced a clear plexiglass cover with openings to access the plate resistors and grid resistors, source points for testing/measuring thru it to the power tube PCB for service. Earlier version was made from thick cardboard notebook, using the power tube PCB cover plate as a template. And calipers to land the rest of the openings.

Sounds like quite a project to restore that amp you've described that tumble-dried itself on tour in the back of the trailer. Shame!!

I have photos of all these mods, just in case. I document everything. A lot of it is in the vast database of MEF's threads under Maintenance, Repair and Troubleshooting Forum where I normally post.

[corliss1]

Awesome, nevetslab - I'll keep all that in mind if he decides to have me do anything!

[nevetslab]

Also, the power amp PCB is prone to have solder joint fractures on the bias pots, preamp out/power amp input jacks, XLR connector. Rear panel PCB parts being used for mechanical support!! Bad Idea! Also the Molex 5-pin power interface connectors on the front edge of the power amp PCB prone to also show solder joint fractures. Then, scan the rest of the surfaces for same. Output Connector PCB also prone for solder joint fractures, AC Power PCB...Fuse connector, and IEC Input connector likewise. Preamp board...well, you've already stated finding broken connections on the pots. Rear of that board is mounted, and not much stress on the front, unless let get bashed around in the back of a truck as it did.